There is presently a great deal of interest in the use of a compacting press to manufacture parts from iron powders and other powders (such as nonferrous powders, ceramic powders, and the like) where each particle of powder is coated with a very thin layer of polymer. If these powders are compacted at temperatures ranging from about 450.degree. F. to about 550.degree. F., the polymer materials "set" and the compacted parts have sufficient strength to eliminate the need for sintering after the compacting operation, although sintering can be practiced if desired.
In conventional compacting operations utilizing uncoated iron powder or the like, the powder is directed from a storage hopper by means of a flexible hose to the hopper of a delivery shuttle by which the powder is shifted to a die cavity in a measured amount. Difficulties are encountered, however, when attempting to use the same sort of delivery system for polymer coated powder wherein the powder is delivered to a heated die and held for a time sufficient to heat the powder to its desired temperature before the compacting operation. First of all, this approach is very time consuming. In addition, it causes localized over-heating where the powder is in contact with the heated die. The polymer coating acts as a temperature insulator, and those coated particles at the interior of the mass are slow to heat, while the polymer-coated particles adjacent the heated die "set" before the interior particles are hot enough to compact. Attempts have also been made to heat the polymer coated powder by external means to a temperature just below the "set" temperature prior to loading the polymer coated powder into the die cavity. It is characteristic of the polymer coated powder that it starts to coagulate and become "tacky" at a temperature of about 350.degree. F. Since the usual production compacting method requires accurate gravity-controlled filling of the powder into the die cavity from a delivery shuttle, any coagulation or tackiness of the coated powder causes variations in the amount of powder that actually is deposited in the die cavity.
U.S. Pat. No. 5,213,816 teaches a delivery system which overcomes a number of these problems. The delivery system heats each polymer coated particle to a temperature just below the coagulation point. The heated powders are accurately fed to a heated die cavity on a conventional compacting press. The remaining increase in temperature to the "set" point is then rapidly achieved during a standard compacting cycle by a combination of the heated tooling and the energy imparted during the actual compacting stroke.
The teachings of the above-noted U.S. Pat. No. 5,213,816 are incorporated herein by reference. Briefly, this patent teaches the use of at least one auger assembly and at least one shuttle assembly. The at least one auger assembly comprises a vertical inlet pipe and a horizontally oriented auger conveyor. The auger conveyor has an entry port and a discharge end. The vertical inlet pipe has an upper end releasably connected to a source of polymer coated powder. The vertical inlet pipe has a lower end connected to the entry port of the horizontal auger. The vertical inlet pipe has a heating element wrapped about its periphery. The auger conveyor has a first heating element wrapped around its periphery and extending from its entry port toward its discharge end and a second heating element extending from the first heating element to the auger conveyor discharge end. The vertical inlet pipe heater imparts heat to the polymer coated powder. The auger conveyor heaters impart additional heat to the individual powder particles through a mixing action. The heated particles from the auger conveyor are discharged into the heated hopper of the shuttle. The shuttle has a heated powder ring which receives a measured amount of heated powder from the shuttle hopper. This measured amount of powder is discharged from the shuttle mechanism into the heated cavity of the compacting press. Thus, the heaters of the vertical tube, the auger conveyor, the shuttle hopper and the shuttle powder ring heat the individual particles of the polymer coated powder to a temperature just below the coagulation point thereof. As indicated above, the "set" point temperature is rapidly achieved during the compacting cycle by a combination of the heated tooling and the energy imparted during the actual compacting stroke.
While the system set forth in U.S. Pat. No. 5,213,816 works well, it would be advantageous to have an even greater throughput of heated polymer coated particles from the auger conveyor. The present invention is based upon the discovery that a greater throughput of the conveyor and more thorough heating of the individual polymer coated particles with greater control can be achieved if, in addition to the external heating elements about the conveyor, the auger is also heated internally. This is accomplished through the use of heated compressed air. When the auger is stopped, compressed air, unheated or heated to a lesser degree, can be used to cool the auger and the polymer coated powder within the auger, to keep the polymer coated powder from melting.